[FIX] web: FieldX2Many must use AbstractManyField is_false method

If the method return false, when a user write require="1" on a x2many field, the content is valid that there have none or a value.

The method was overwrited for a visual change (display the field x2m in readonly mode when they are not value) but is_false is priority use to ensure datas.

addons/web/static/src/js/views/form_relational_widgets.js

@@ -863,9 +863,6 @@ var FieldX2Many = AbstractManyField.extend({
         }
         return true;
     },
-    is_false: function() {
-        return false;
-    },
 });
 
 var X2ManyDataSet = data.BufferedDataSet.extend({